Having AI Accelerate Up & Down

So I'm working on Pong, just something simple to get myself introduced to Unity and I'm on the last bit of the gameplay (next up menus!).

Anyway, I was just wondering, how can I tell the AI to accelerate up to top speed and down to bottom speed just to get the movement to be a heck of a lot more smooth?

Here's what I have so far:

 using UnityEngine;
 using System.Collections;
 
 public class AIScript : MonoBehaviour {
 
     //Create a magnitude for the paddles velocity later
     public float speed = 5.0f;
     //Create a ball object to follow the height value of
     public GameObject ball;
     //Create a paddleheight variable to allow for simple calculations laterS
     private float aiHeight;
     //Set up a temporary RacketDistance value to keep the FixedUpdate solution more simple.
     private float tempRacketDist;
     //Set private bool to determine previous racket direction
     private bool velup;
 
     //Called on Instantiation of the AI Racket
     void Start() {
         aiHeight = GetComponent<Renderer> ().bounds.size.y;
     }
 
     void FixedUpdate() {
         //IF the ball is travelling TOWARD the AI Racket
         if (ball.GetComponent<Rigidbody2D> ().velocity.x > 0) {
             //Find out the Racket Distance Value
             RacketDistance();
             //If it's positive
             if(tempRacketDist > (0.5f*aiHeight)) {
 
 
                 GetComponent<Rigidbody2D>().velocity = Vector2.up * speed;
                 velup = true;
 
 //Otherwise it's in the negative direction
             } else if (tempRacketDist < (-0.5f*aiHeight) ) {
 
 
                 GetComponent<Rigidbody2D>().velocity = Vector2.down * speed;
                 velup = false;
 
 //If the ball is in the centre of the racket
             } else if ( tempRacketDist == 0 || tempRacketDist < (0.5f*aiHeight) && tempRacketDist > (-0.5f*aiHeight) ) {
                 //If last direction was up
                 if(velup) {
                     //Continue up but have 1/3 speed
                     GetComponent<Rigidbody2D>().velocity = Vector2.up * (0.3f * speed);
                 //If last direction was down
                 }else{ 
                     //Continue down but at 1/3 speed
                     GetComponent<Rigidbody2D>().velocity = Vector2.down * (0.3f * speed);
                 }
                 //GetComponent<Rigidbody2D>().velocity = Vector2 * (1/5*speed);
 
             }
         } else {
             //IF the ball isn't travelling TOWARD the AI Racket, set it's velocity to zero
             this.GetComponent<Rigidbody2D>().velocity = Vector2.zero;
         }
     }
 
     void RacketDistance() {
         //Positive value means ball is above the mid-point of the racket
         //0 value means ball is at the centre of the racket, just return it with a y=0 vector
         //Negative value means ball is below the mid-point of the racket
         //
         //NOTE - This is a repurposed rip-off of the HitFactor function in the BallScript.cs file
         tempRacketDist = (ball.transform.position.y - this.transform.position.y); 
     }
 }